Cavitational drilling utilizing an acoustic generator and an acoustic concentrator



United States Patent llll 3,545,552

3,251,424 5/1966 Brooks inventor Frank A. Angona Dallas, Texas Appl. No.783,226 Filed Dec. 12, 1968 Patented Dec. 8, 1970 Assignee Mobil OilCorporation a corporation of New York CAVITA'IIONAL DRILLING UTILIZINGAN ACOUSTIC GENERATOR AND AN ACOUSTIC CONCENTRATOR 3 Claims, 3 DrawingFigs.

u.s. Cl 175/65, l66/ l 77 Int. Cl E21b7ll8 Field olSearch l75 /55-References Cited UNITED STATES PATENTS 3,302,720 2/1967 Brandon l75/56X3,405,770 10/1968 Galle et al... l75/65X 3.438.352 4/1969 Brandon 166/177X Primary Examiner-David H. Brown Atlorneys--William J. Scherback,Frederick E. Dumoulin,

William D. Jackson, Andrew L. Gaboriault and Sidney A. Johnson ABSTRACT:This specification discloses an improved method and apparatus fordrilling a liquid-filled borehole into the earths crust withcavitational energy. Acoustic energy is generated at a first region inthe borehole of an amplitude less than that required to producecavitation in this region. This acoustic energy is then focused to asecond region downhole of the first region and concentrated to anamplitude greater than the amplitude required to produce cavitation inthe second region. The second region is moved about the bottom of theborehole to effect cavitational drilling of the borehole.

CAVITATIONAL DRILLING UTILIZING AN ACOUSTIC GENERATOR AND AN ACOUSTICCONCENTRATOR BACKGROUND OF THE INVENTION This invention relates to thedrilling of boreholes into the earths crust. More particularly, itrelates to the drilling of such boreholes by using acoustic energy toeffect cavitation in the drilling liquid and utilizing the resultingshock waves from the cavitation to fracture earth materials at thebottom of the borehole. 1

Numerous techniques are available for drilling boreholes into the earthscrust. A relatively new drilling technique which is showing increasingpromise utilizes cavitation of a liquid within the borehole. Cavitationis a phenomenon whereby under certain conditions cavities form andviolently collapse within a liquid. A shock wave results from thecavitation and may cause considerable mechanical damage to neighboringsolid surfaces. Cavitation takes place within a liquid when the pressurewithin the liquid is reduced by an amount commonly termed thecavitational threshold. The cavitational threshold is a function of thepressure on the liquid and is also influenced by various other factorssuch as the presence of gas or solid particles which serve as nuclei forcavitation. The cavitational threshold is the value of the acousticpressure required to produce cavitation and in general the cavitationalthreshold of a drilling liquid at a particular location within aborehole -is somewhat less than the hydrostatic pressure of the drillingliquid at this location.

Normally in cavitational drilling, a drilling liquid is circulateddownward through a drill string and thence upward to the surface of theearth through the annulus formed between the outer wall of'the drillstring and the borehole wall. Cavitation is induced in the drillingliquid by any suitable technique such as by the generation of acousticenergy. The resulting shock waves function to break up the rock surfacesat the bottom of the borehole. The rock fragments thus produced areentrained in the circulating drilling liquid and withdrawn from theborehole with this liquid.

Various cavitational drilling techniques are known in the art. Forexample, in U.S. Pat. No. 3,315,755, to W. B. Brooks,

there is disclosed a technique for carrying out cavitational drillingthrough the use of acoustic energy. In this technique, a piston isvibrated to impart acoustic energy to the drilling liquid. The acousticenergy induces cavitation in the drilling liquid and shock wavesresulting therefrom are utilized in drilling the borehole. Anothertechnique in which acoustic energy is utilized to efiect cavitation in adrilling liquid is disclosed in U.S. Pat. No. 3,387,672,-to E. l...Cook. In this technique, the cavitational threshold of the drillingliquid an amount of gas which goes into solution therein.

SUMMARYOF THE INVENTION In accordance with the present invention thereare provided new and improved methods and systems for practicingcavitational drilling. In practicing the method of the presentinvention, drilling liquid is circulated through the borehole. Acousticenergy is generated at a first region inthe borehole of an amplitudeless than the amplitude required to produce cavitation in the drillingliquid in this first region. This acoustic energy is 'focused'through aliquid path to a second or focal region in the borehole and concentratedto a second amplitude greater than the amplitude required to producecavitation in the drilling liquid in this second region. Cavitation isthus produced in this second region and the resulting shock waves areutilized to break up'earth materials at the bottom of the borehole.

In accordance with another embodiment of the invention, there isprovided apparatus for drilling a borehole by cavitational energy. Thisapparatus is comprised of an elongated housing having a passagewaytherein for the circulation of drilling liquid. Included in the housingare a flexible coupling, an acoustic transducer, focusing means locatedbelow the coupling and transducer for focusing acoustic energy generatedby the transducer through a path extending below the housing, anddeflector means below the coupling for moving the focusing meanslaterally within the borehole. BRIEF DESCRIPTION OF THE DRAWINGS;

FIG. 1 is an illustration showing a drilling tool formed in accordancewith one embodiment of this invention and located in a borehole;

FIG. 2 is a cross-sectional view of a biconical concentrator attached toan acoustic transducer and located in a borehole; and

FIG. 3 is an illustration of the bottom of the borehole showing a traceof the path that the focal region follows over the bottom of theborehole during operation of the drilling tool.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, there isshown a borehole tool, more specifically a cavitational drilling tool,attached to a drill string located in a borehole. A fluid passageway isprovided through the drill'string and drilling tool whereby drillingliquid may be circulated downward through the drill string and upwardthrough the annulus formed between the drill string and the wall of theborehole. An acoustic transducer or generator is actuated by thecirculating drilling liquid to vibrate a piston and produce acousticenergy within the drilling liquid at a first region in the vicinity ofthe piston. This acoustic energy is of a first amplitude which is lessthan'the amplitude required to produce cavitation in the drilling liquidin this first region. The acoustic energy is concentrated by a focusingmeans to a second region whereby cavitation'is produced in the drillingliquid. The drilling tool further includes means for moving the focusingmeans laterally within the borehole whereby the second region is movedover the bottom of the borehole for improved drilling efficiency.

More particularly and with reference to FIG. 1, borehole 3 is shownextending into the earths crust 2. This borehole is initially startedand drilled into the earths crust by any suitable technique, such as arotary procedure, for at least a depth greater than the length of thecavitational drilling tool of this invention. Normally the borehole willbe drilled to some depth at which it becomes more economical to continuedrilling the borehole with cavitational energy rather than by rotarytechniques. The rotary drilling tool is then withdrawn from the boreholeand the cavitational drilling tool of this invention is attached to thelower end of the drill string and lowered into the borehole.

In FIG. 1 a cavitational drilling tool is shown positioned in thevicinity of the bottom of borehole 3 and secured to drill string 11. Thecavitational drilling tool is comprised of an elongated housing 1 havinga passageway therein for the circulation of drilling liquid 6. Thefollowing elements of the cavitational drilling tool are secured intoelongated housing 1 by, for example, threaded connections; flexiblecoupling 9, acoustic transducer or generator 5, focusing means 7, anddeflector means which include deflector 10 keyed into grooves 8 whichare formed in a tapered portion of the outside case of elongatedhousing 1. Focusing means 7 is located in housing 1 below flexiblecoupling 9 and acoustic transducer 5. The deflector means is located inhousing 1 below flexible coupling 9.

In a preferred cavitational drilling tool, flexible coupling 9 forms theupper part of housing 1 and is adapted at its upper end for connectingto a drill string. Acoustic transducer 5 is secured to the lower end ofcoupling 9 and focusing means 7 is secured to the lower end oftransducer 5. The outside case of focusing means 7 is tapered andgrooves 8 are formed in it. Deflector I0 is keyed into grooves 8 tocomplete the deflector means. When this preferred embodiment is utilizedin drilling a borehole, piston 12 is located immediately adjacent theupper end of the focusing means and the deflector means moves thefocusing means laterally within the borehole by flexing coupling 9.

means 7. Also, the deflector means maybe located at other positionsinhousingl than .in the outside case of focusing means 7 so long as itis located belowcoupling means 9.

Acoustic generators "may beanytype of transducer which" iscapable ofbeing operated to produce acoustic energy at the first region in theborehole of afirst amplitude that is lessthan the amplitude to. producecavitation in the drilling liquid within-the borehole at thisfirstregion. Stated otherwise, this'first amplitude is lessthan thecavitational threshold of the drilling liquid in the first-region.

In the preferred -.ernbodiinent of the invention illustrated, theacoustic generator '5 comprises a vibrating piston 12 which is driven bya turbine (notshown). By circulating drilling liquid through the drillstring 11 and housing 1, the turbine is actuated andvibrates piston 12thereby generating acoustic energyin' the drilling liquid. Suitableacoustic generators including a vibratory piston forusein the presentinvention.arethe turbine-powered acoustic pressure pulse generators andjet: edge generators disclosed in US. Pat. No. 3,315,755 to'B'rooks:

A suitable flexible coupling means for use in the invention is on e'coniprised of reinforcedflexible'hydraulic hose lined on the inside witha-collapse-r'esisting spring and enclosed within a burst-resistantspring. For a' detail description ofxsuch a I coupling reference is madetofNew Concept in Drilling Rigs Designed to Lower wen Costs, wonmoin114, Dec. 1967: I

.ln drilling a welliin accordance with thewpresent invention; drillingliquid 6 is. circulated downward through drill string 11 andupwardthtough annulusd, defmedby the outer wall of drill string lland-the borehole 3.'Acoustic generator is actuated bytht: circulatingdrilling liquid. to produce at a first region in the borehole acousticenergy of afi'rst amplitude that is less-than thamplitude required toproduce cavitation in the drilling liquidin' this first region. Theacoustic energy at this first region'is then focused by focusing means 7to a second or focal region 13 located. at approximately the bottom ofborehole 3. The acoustic energy 'is' concentrated by the focusing'meansatthe second region 'and'the amplitude is thus increased to a secondamplitude which is greater than. the amplitude requiredto producecavitation inthe drilling liquid in this second region: Cavitationv isthus produced within this secondr'egion'andthe shock waves resultingfrom this cavitation are utilizedto :break up theearth materials at thebottom of the borehole;

The focusing means 7 may be of any suitable type but preferably takesthe forrn'of'a biconical concentrator 30, as shown'in FIG. 2.' referenceto FIG. 2, the biconical concentrator 'is attached bythreads 32 toacoustic generator 5 and positioned in the lower portion of borehole 3.The biconical concentrator is comprised of two conical frusta 34 and 36joined end-to-end by-their smaller apertures at 38. Acoustic energyproducedby acoustic generator 5 is illustrated by ray paths 40. Thisacousticenergy is concentrated by the biconical concentrator to a focalpoint 14 within a second region 13v in the vicinity of the boreholebottom. The amplitude of the acoustic energyproduced'by acousticgenerator 5 at the first region in the borehole is thusrincreased to asecond amplitude in focal region. 13"and cavitation iseffectedin thedrilling liquid. The importance offocusing and concentrating this energyin a'secondregion is twofold. First, the amplitude of theacousticenergy-generated by the acoustic generator is increased,th'ere'byimproving the efficiency of the generator particularly fordrilling deep boreholes; and second, the region of cavitation is removedfrom the immediate vicinity of the acoustic generator-,"thereby avoidingexcessive wear of the The deflector. functions to move the focusing meamlaterally within the borehole. Spiral grooves 8 are provided in theoutside surface of tapered biconical concentrator 30. Deflector means 10is slidably keyed in grooves 8, and rubs against the wall of borehole 3.when deflector-means 10 is at the highest position of spiral grooves 8,focal region 13 extends to its most lateral position in borehole 3 wherecavitation in this region functions totbreak up the earth materialalong-the outside edges of the borehole. When deflector 'means 10 isinits lowermost position in spiral grooves 8, focal region 13 iscentered about a point 14 which is offset a short lateral distance fromthe longitudinal axis of the borehole. Thus, flexible coupling means 9biases the cavitational drilling .tool toward the longitudinal axis ofthe borehole while deflector means 10 biases it toward the wall thereof.

In operation, drilling liquidis circulated through elongated housing 1and then upward through annulus M0 the surface of the earth. Acousticenergy is produced at a first region by acoustic generator 5 whichacoustic energy is focused to a focal region 13 where cavitation isinitiated. Rotational torque is applied to drill string '1 l byconventional drilling machinery (not shown). This Rotational torque istransmitted by flexible couplingmeans 9 to the cavitational drillingtool causing it to be rotated. This rotation of the cavitationaldrilling tool and differential friction between deflector means 10 andthe wall of borehole 3 cause deflector means 10 to move upwardv anddownward along spiral grooves 8,'thereby laterally moving focusing 7within the borehole. Concomitantly, flexible coupling 9 exerts. acenteringforcewhich tends to center zfocusing means 7 along thelongitudinal axis of borehole 3'whiledeflector means .10 produces alateral offset of focusing means 7 'from'the longitudinal axis ofborehole 3 even when deflector means 10 is in its lowermost positionin-v spiral grooves 8. The result of these forces-and counteractingforces is that focusing means 7'is moved laterally within the boreholeconcomitantly? with the rotation of the drill string. Thus focalregion'13 is moved about the bottom of borehole 3 to effect drilling ofthe borehole.

Referring to FIG. 3,"there is illustrated a plan view of the bottom ofborehole 3 .having superimposed thereon a trace of the path that focalregion 13 follows as it sweeps the borehole bottom. Focal point 14,which is the center of focal region :13, is shown offset a shortdistance from the center of borehole 3. As the cavitational drillingtool is rotated, deflector means 10 sliding in spiralgrooves S-laterallymoves focusing means 7 within the borehole 3 and causes the focal region13 to be moved about the bottom of the borehole in a spirallike patternindicated by broken line 15 until deflector means loreaches thehigherrnost point of spiral grooves 8. Focal point 14 is then located ata laterally displaced position indicated, by reference numeral 16.'Thereafter, with continual rotation the'trace of focal region 13.spiralsback toward the center of borehole 3 .until deflector means 10 reachesits lowermost position and.

focal. point 14 is again in its initial position or in a similarlaterally offset position from the center of borehole 3.

I claim:

1. A method of drilling a borehole into the earths crust focusing saidacoustic energy through a liquid path to a second region in saidborehole to concentrate said acoustic energy to a .second amplitudegreater thanthe amplitude required to produce cavitation in saiddrilling liquid in said second region, whereby cavitation is producedwithin said second region to break up earth material at the bottom ofsaid borehole.

' material at said bottom of said borehole.

3. The method of claim 2 wherein said second region is moved over thebottom of said borehole to trace a spirallike pattern.

